In a typical audio and video decoding system, an incoming signal, such as a transport stream, is provided to a demultiplexor. The transport stream includes any type of stream capable of carrying one or more embedded signals of audio, video, data, or a combination thereof. More specifically, a typical transport stream provides a plurality of channels of data similar to the different channels within a television set, wherein the transport demultiplexor demultiplexes the transport stream parsing out a specific channel. More specifically, a typical transport demultiplexor generates an audio signal, a video signal, and a program clock reference (PCR) signal from the transport stream. The audio signal is provided to an audio decoder, such as any device implemented either in hardware or software or a combination thereof, wherein the audio information is converted into audible format. The video information is provided to a video decoder which includes any device, implemented either in hardware, software or a combination thereof, wherein the information from the video stream is decoded and provided to a video output such as a display or a television.
Further within the video decoding system, the program clock reference signal is provided to a system time clock (STC) counter. The STC counter provides timing information to the audio decoder and the video decoder, whereupon the decoder utilizes the timing information with regards to the audio and video signals from the transport demultiplexor, to ensure the synchronization of the audio and video decoder system. Utilizing the example of an MPEG decoding system, the timing information has a frequency base of 27 MHz. The decoding system also requires a frequency signal for audio clocks, wherein the frequency may not necessarily be generatable from a 27 MHz source. Therefore, the decoding system requires separate timing signals having different frequencies.
One common solution is to provide a phase locked loop (PLL) with a single voltage source to generate different frequencies such as audio frequencies for an audio decoder and STC clock frequency, wherein the voltage source provides the PLL the voltage signal that would be adjusted to the second set of frequencies and provided to the STC counter and the decoder, such as found in the Xilleon ™ 220 sold by ATI Technologies, Inc., 1 Commerce Valley Drive East, Markham, Ontario. For instance, the generation of these difference frequencies may be accomplished using one or more frequency adjustors in combination with the PLL. In one example, the PLL output's frequency may be adjusted by any suitable frequency adjustor to generate a different frequency for one of the audio decoder and STC clock frequency. When the transport stream is encoded in an encoding scheme which provides an STC timing counter with a base extension, the PLL generates the STC signal to drive the STC counter having the appropriate frequency to coordinate with the PCR signal. Although, when the transport stream is encoded using an encoding scheme that does not provide for an STC timing counter with a base extension, the PLL is unable to produce proper driving signals for the STC counter and the decoder at the same time. This scenario provides for the possibility of a loss of synchronization between the STC counter and the PCR which thereupon may cause audio and video interruption. In one embodiment, a DirecTV(®) transport stream may provide encoded video data having an encoding scheme such that the prior art decoder would only be able to synchronize and decode the encoded audio and video having the STC timing counter with the base extension.
Another proposed solution is to provide multiple PLL voltage sources to generate the audio frequencies and STC clock frequency. This solution increases manufacturing costs by requiring a separate PLL and also decreases the amount of available on-chip silicon as separate PLLs require a greater amount of silicon area on-chip. Another complication found with video decoding systems is that some transport streams have multiple encoding schemes, whereupon different approaches to the timing of STC counter must be utilized. Therefore, simply providing a system with multiple PLLs may not accommodate an incoming transport stream having several different types of encoded information.
As such there exists a need for an improved audio and video decoding system whereupon multiple timing signals having varying frequencies may be generated utilizing a single PLL and the system may further accommodate different encoded transport streams.